Cellular and subcellular effects of very heavy ions

The biological effects of irradiation with ions of masses larger than 40 and energies up to 20 MeV per atomic mass unit are reviewed. The objects are viruses, bacterial spores, yeast and mammalian cells. Experimental parameters include loss of colony forming ability, induction of mutants, chromosomal aberrations, cell cycle progression, inhibition of biochemical activities and the formation of strand breaks. Some of the pertinent physical questions--e.g. track structure--are also discussed. It is shown that with very heavy ions the biological effectiveness is no longer unambiguously related to a single parameter like l.e.t. or Z*2/beta 2 but depends strongly on ion energy. This points to the importance of far-reaching delta-electrons. The analysis indicates also that even with very high l.e.t., cells are not killed by the passage of a single particle through their nucleus. Possible implications of the findings for fundamental radiation biology are outlined.     

Cellular responses to excess phospholipid

Phosphatidylcholine (PtdCho) is the major membrane phospholipid in mammalian cells, and its synthesis is controlled by the activity of CDP:phosphocholine cytidylyltransferase (CCT). Enforced CCT expression accelerated the rate of PtdCho synthesis. However, the amount of cellular PtdCho did not increase as a result of the turnover of both the choline and glycerol components of PtdCho. Metabolic labeling experiments demonstrated that cells compensated for elevated CCT activity by the degradation of PtdCho to glycerophosphocholine (GPC). Phospholipase D-mediated PtdCho hydrolysis and phosphocholine formation were unaffected. Most of the GPC produced in response to excess phospholipid production was secreted into the medium. Cells also degraded the excess membrane PtdCho to GPC when phospholipid formation was increased by exposure to exogenous lysophosphatidylcholine or lysophosphatidylethanolamine. The replacement of the acyl moiety at the 1-position of PtdCho with a non-hydrolyzable alkyl moiety prevented degradation to GPC. Accumulation of alkylacyl-PtdCho was associated with the inhibition of cell proliferation, demonstrating that alternative pathways of degradation will not substitute. GPC formation was blocked by bromoenol lactone, implicating the calcium-independent phospholipase A2 as a key participant in the response to excess phospholipid. Owing to the fact that PtdCho is biosynthetically converted to PtdEtn, excess PtdCho resulted in overproduction and exit of GPE as well as GPC. Thus, general membrane phospholipid homeostasis is achieved by a balance between the opposing activities of CCT and phospholipase A2.     

Cellular responses to enteropathogenicEscherichia coli infection

EnteropathogenicEscherichia coli (EPEC), first described in the 1940's and 1950's, remain an important cause of severe infantile diarrhoea in many parts of the developing world. EPEC do not produce enterotoxins and are not invasive; instead their virulence depends upon exploitation of host cell signalling pathways and the host cell cytoskeleton both as a means of colonizing mucosal surfaces of the small intestine and causing diarrhoea. Following initial mucosal attachment, EPEC secrete signalling proteins and expresss a surface adhesin, intimin, to produce attaching & effacing lesions in the enterocyte brush border membrane characterised by localised destruction of brush border microvilli, intimate bacterial adhesion and cytoskeletal reorganisation and accretion beneath attached bacteria. The pathophysiology of EPEC diarrhoea is also complex and probably results from a combination of epithelial cell responses including both electrolyte secretion and structural damage.     

Cellular responses to extracellular guidance cues

Extracellular guidance cues have a key role in orchestrating cell behaviour. They can take many forms, including soluble and cell‐bound ligands (proteins, lipids, peptides or small molecules) and insoluble matrix substrates, but to act as guidance cues, they must be presented to the cell in a spatially restricted manner. Cells that recognize such cues respond by activating intracellular signal transduction pathways in a spatially restricted manner and convert the extracellular information into intracellular polarity. Although extracellular cues influence a broad range of cell polarity decisions, such as mitotic spindle orientation during asymmetric cell division, or the establishment of apical–basal polarity in epithelia, this review will focus specifically on guidance cues that promote cell migration (chemotaxis), or localized cell shape changes (chemotropism).     

Plant responses to extracellular nucleotides: Cellular processes and biological effects

Higher plants exhibit cellular responsiveness to the exogenous application of purine nucleotides in a manner consistent with a cell–cell signaling function for these molecules. Like animals, plants respond to extracellular ATP, ADP, and stable analogues (e.g., ATPγS and ADPβS) by increasing the cytoplasmic concentration of calcium. Agonist substrate specificity and concentration dependency suggest receptor mediation of these events, and, although the identity of the plant receptor is currently unknown, pharmacological analysis points to the involvement of a plasma membrane-localized calcium channel. Extracellular ATP can also induce the production of reactive oxygen species and stimulate an increase in the mRNA levels of a number of stress- and calcium-regulated genes, suggesting a role for nucleotide-based signaling in plant wound and defense responses. Furthermore, the growth and development of plants can also be altered by the application of external ATP. Recent studies are only beginning to uncover the complexities of plant signaling networks activated in response to extracellular ATP and how these might interact to affect plant physiological processes.     

Cellular responses to endoplasmic reticulum stress and apoptosis

The endoplasmic reticulum (ER) is the cell organelle where secretory and membrane proteins are synthesized and folded. Correctly folded proteins exit the ER and are transported to the Golgi and other destinations within the cell, but proteins that fail to fold properly—misfolded proteins—are retained in the ER and their accumulation may constitute a form of stress to the cell—ER stress. Several signaling pathways, collectively known as unfolded protein response (UPR), have evolved to detect the accumulation of misfolded proteins in the ER and activate a cellular response that attempts to maintain homeostasis and a normal flux of proteins in the ER. In certain severe situations of ER stress, however, the protective mechanisms activated by the UPR are not sufficient to restore normal ER function and cells die by apoptosis. Most research on the UPR used yeast or mammalian model systems and only recently Drosophila has emerged as a system to study the molecular and cellular mechanisms of the UPR. Here, we review recent advances in Drosophila UPR research, in the broad context of mammalian and yeast literature.     

Cellular responses to naphthoquinones: juglone as a case study

The effects of juglone (JG) on the endogenous growth, growth in the presence of either indoleacetic acid (IAA) or fusicoccin (FC) and on proton extrusion were studied in maize coleoptile segments. In addition, membrane potential changes were also determined at chosen JG concentrations. It was found that JG, when added to the incubation medium, inhibited endogenous growth as well as growth in the presence of either IAA or FC. Simultaneous measurements of growth and external pH indicated that inhibition of either IAA-induced growth or proton extrusion by JG was a linear function of JG concentration. Addition of JG to the control medium caused depolarization of the membrane potential (E_m), value of which was dependent on JG concentration and time after its administration. Hyperpolarization of E_m induced by IAA was suppressed in the presence of JG. It was also found that for coleoptile segments initially preincubated with JG, although subsequently removed, addition of IAA was not effective in the stimulation of growth and medium acidification. Taken together, these results suggest that the mechanism by which JG inhibits the IAA-induced growth of maize coleoptile segments involves inhibition of PM H⁺-ATPase activity.     

Cellular responses to extreme water loss: The water-replacement hypothesis

The previously advanced hypothesis that desiccation resistance involves the replacement of water adjacent to intracellular surfaces with polyhydroxy compounds has been supported by experiments on cysts of the brine shrimp, Artemia, and in a model system of albumin-glycerol-water, using nuclear magnetic resonance spectroscopy, microwave dielectrics, and density measurements. We have also considered other problems that cells face when large fractions of their total water content are removed. Observations by other investigators have indicated that a variety of mammalian cells can lose roughly 50% of their water and survive; for a given cell type death occurs if its volume is reduced below a certain minimum level. Membrane damage has previously been suggested to be a major cause of dehydration damage. We have proposed some additional plausible mechanisms that might also be involved.     

Cellular responses to replication stress: Implications in cancer biology and therapy

DNA replication is essential for cell proliferation. Any obstacles during replication cause replication stress, which may lead to genomic instability and cancer formation. In this review, we summarize the physiological DNA replication process and the normal cellular response to replication stress. We also outline specialized therapies in clinical trials based on current knowledge and future perspectives in the field.     

Plant cell responses to heavy metals: molecular and physiological aspects

The effect of lead, cadmium and cooper on protein pattern, free radicals and antioxidant enzymes in root of Lupinus luteus L. were investigated. Heavy metals inhibited growth of lupin roots, which was accompanied by increased synthesis and accumulation of a 16 kDa polypeptide (Przymusiński et al. 1991 Biochem. Physiol. Pflanzen., 187:51–57). This component has been earlier identified as immunologically related to Cu,Zn-superoxide dismutase (Przymusiński et al. 1995 Env.Exp.Bot., 35:485–495). However, more detailed study revealed that this stress-stimulated protein is composed of four to six polypeptides of different electrophoretic mobility. The most abundant polypeptides of the 16kDa region were found to be closely homologous to pathogen related proteins. The number and intensity of these polypeptides was highly variable in roots of individual seedlings, which suggests that they might represent separate allelic forms. Electron paramagnetic spectra revealed that at low lead concentrations the amplitude of the first derivative was similar to the control and distinctly increased at higher metal concentrations. On the other hand, at the lower lead concentrations the activity of antioxidant enzymes increased, whereas at higher metal doses the enzyme activities did not raise further (SOD) or even dropped (CAT, APOX). This implies that the responses of antioxidant system to lead is dose-dependent stimulated by low metal concentrations, whereas at the higher metal level the free radical emission is beyond the quenching capacity of antioxidant enzymes, which in turn might contribute to the reduced root growth. The effect of various heavy metals: Pb²⁺, Cd²⁺ and Cu²⁺ on phytochelatins and antioxidant enzymes depends on the kind of metal ion. Pb²⁺ and Cd²⁺ stimulated the PCs formation whereas Cu²⁺ was not effective. On the other hand, in root exposed to Cu the activity of catalase (CAT) was the highest as was the production of H₂O₂. The strong oxidative effect of Cu²⁺ ions which were not complexed by PCs suggests that these peptides might by involved in the cellular defense system by binding excessive heavy metal ions. On the basis of our results it can be concluded that in lupin roots exposed to heavy metals there is a complex defense system against metal phytotoxicity, which comprises of specific proteins, antioxidant enzymes and phytochelatins.     

Cellular and genetic responses to mesoderm induction in Xenopus

Mesodermal cell differentiation begins in response to an inductive interaction early in frog development. In parallel with the recent finding that certain peptide growth factors can induce mesoderm, early cellular and genetic responses to the induction have been discovered. I review here recent work on these responses, work that aims to understand how cells respond to inducers to form the complex pattern of the vertebrate mesoderm.     

Neuromuscular responses to explosive and heavy resistance loading

The EMG power spectrum may shift towards higher frequencies with higher movement velocities. Fatigue, on the other hand, can cause a decrease in the frequency components. The purpose of this study was to examine acute effects of explosive (EE) and heavy resistance (HRE) concentric leg press exercise on muscle force, EMG and blood lactate. The EE included five sets of ten repetitions with 40±6% of the isometric maximum at a 100° knee angle performed as explosively as possible. The same number of repetitions was performed in HRE but with a heavier load (67±7% of the isometric maximum at a 100° knee angle). Maximal isometric and single concentric actions of different loads, and an isometric fatigue test were measured before and after both exercises. Surface EMG was recorded from the vastus medialis muscles for analyses of average EMG (aEMG) and EMG power spectrum. Muscle fiber composition of the vastus lateralis was determined and blood lactate measured throughout the exercises. Mean power frequency and median frequency were higher during EE than during HRE (P<0.05). They increased during EE (P<0.05) as the exercise progressed, whereas during HRE no change or even slight decreases were observed. Signs of fatigue after pure concentric work were not observed after EE, and even after HRE, possibly due to the relatively small range of motion and short duration of action time, the fatigue was not that extensive. The relative number of fast twitch fibers was correlated (r=0.87, P<0.05) with the change in blood lactate in HRE. It was concluded that there may be a greater use of fast twitch motor units in explosive movements and that instead of fatigue, the present number of concentric actions in explosive exercise seems to have facilitated the neuromuscular system.